Chiara Elmi

Mineralogical and compositional features of rock fulgurites: A record of lightning effects on granite

Abstract Fulgurites are a naturally occurring glass formed when sand, rock, or soil is struck by atmospheric electrical discharges (lightning). The aim of this paper is to provide insights into the conditions occurring in rocks during the lightning strike. Rock fulgurites collected from Mt. Mottarone, Baveno (Piedmont, Italy) have been investigated to assess the mineralogical and compositional changes occurring in granite due to a lightning strike. X-ray powder diffraction showed that the samples represent the dominant granitic rock type of the Baveno massif, the so-called “Pink Baveno.” Fulgurite coats the surface of the granite as a brown-black, glassy to very fine-grained porous layer. Powder diffraction data for the fulgurite reveal the presence of cristobalite and quartz crystals in a glass matrix, suggesting that temperature exceeded ~1700 °C at near atmospheric conditions, assuming thermodynamic equilibrium. Electron probe microanalysis of the glass revealed that it is mainly composed of SiO2 and Al2O3 and that it has a porosity of 5–7 area% in the studied zones. The presence of the amorphous phase indicates that the abrupt electrical (Joule) heating of the rock surface yielded high temperatures, producing a thin melt layer on the surface, which then cooled adiabatically. Idealized physical model was developed to simulate the effects of Joule heating and subsequent thermal conduction close to the rock surface during and after a lightning strike. The quantity of organic matter in the glass, obtained via Elemental Analyzer, suggests that rapid quenching of the melt trapped NOx and COx gases produced during heating. Raman spectroscopy revealed the presence of polyaromatic hydrocarbon molecules, which, combined with the Elemental Analyzer data, suggest that organic matter was pyrolyzed at around 300–350 °C and then trapped in the glass matrix of the studied rock fulgurites.

Determination of Hg binding forms in contaminated soils and sediments: state of the art and a case study approaching abandoned mercury mines from Mt. Amiata (Siena, Italy)

This paper, after an introduction about the state of the art in the methods for the evaluation of Hg chemical speciation, focuses on Hg polluted soils and sediments from Siele mining area (Piancastagnaio, Siena, Italy). In particular, the mineralogical composition was evaluated via X-ray powder diffraction, total Hg was determined via inductively coupled plasma optical emis- sion spectroscopy on acid digested samples, and Hg binding forms were investigated both via sequential selective extraction and pyrolytic extraction. Solutions obtained via sequential selective extraction were checked for Hg content using inductively coupled plasma optical emission spectroscopy, whereas to carry out pyrolytic extraction a thermogravimetric analyser coupled with an evolved gas mass spectrometer was used. To our knowledge, this last method was never used for this purpose on natural samples. Experimental results enhance that: i) Hg detection via thermal analyses can qualitatively distinguish among different Hg com- pounds. ii) The temperature of Hg thermal release is affected by both the sample matrix and the Hg bond type. iii) Thermal analyses fail in the attempt of a quantitative determination of the evolved species; this goal is better achieved via the selective chemical leach- ing. iv) Most of the Hg is related to HgS-like bonds, thus pointing to the presence of very stable Hg compounds. v) Only in some samples we found also organo-complexed Hg-like compounds and, in a limited amount, hydrosoluble Hg and/or Hg species soluble in weakly acid environment. vi) No correlation between sampling point and both total Hg and/or Hg speciation was observed.

CRYSTAL CHEMISTRY AND SURFACE CONFIGURATIONS OF TWO IRON-BEARING TRIOCTAHEDRAL MICA-1M POLYTYPES

The crystal chemical features of the bulk and the uppermost (001) surface layers of freshly cleaved surfaces of two trioctahedral Fe-rich mica-1M (space group C2/m) polytypes, i.e. a tetraferriphlogopite from an alkaline-carbonatitic complex near Tapira, Belo Horizonte, Minas Gerais, Brazil, and an Fe2+-bearing phlogopite containing less tetrahedral Fe3+ from the Kovdor carbonatite-bearing, alkaline-ultrabasic complex, Kola Peninsula, Russia, are explored here. Mineral-surface effects were investigated by X-ray Photoelectron Spectroscopy (XPS) and compared to the bulk structure derived from single-crystal X-ray diffraction data. Based on microprobe analysis and the X-ray study, the chemical formulae are [XII](K0.99)[VI](Fe0.082+Fe0.153+Mg2.76Ti0.01)[IV](Fe0.823+Si3.18)O10.37F0.24(OH)1.39 and [XII](K0.94Na0.06)[VI](Fe0.172+Fe0.053+Mg2.75Mn0.01Ti0.05)[IV](Fe0.163+Al0.84Si3.00)O10.21F0.35(OH)1.44 for tetraferriphlogopite and Fe-bearing phlogopite, respectively. The tetrahedrally coordinated sites of the two minerals differ, where Fe-for-Si substitution is at 20.5% in tetra-ferriphlogopite and at 4% in Fe-bearing phlogopite.The bulk study showed that Fe3+ substitution increases the tetrahedral sheet thickness and the mean tetrahedral edge lengths in tetra-ferriphlogopite compared to Fe-bearing phlogopite. The tetrahedral rotation angle (α) changes remarkably from tetra-ferriphlogopite (α = 10.5°) to the Fe-bearing phlogopite (α = 8.5°), thus indicating a significantly greater initial lateral sheet misfit (leading to a greater tetrahedral ring distortion) between the tetrahedral and the octahedral sheets in the tetra-ferriphlogopite compared to Fe-bearing phlogopite. The Fe3+ substitution for Si and the differences in lateral dimensions of the tetrahedral and octahedral sheets affect the tetrahedral flattening angle (τ), with τ = 109.9° for tetraferriphlogopite and τ = 110.7° for Fe-bearing phlogopite.The binding energy (BE) of photoelectron peaks in XPS is dependent on the chemical state of atoms and on their local environment at the near surface. The Mg in both phlogopites is bonded to F, with the BE of Mg1s increasing as coordinated oxygen atoms are substituted by fluorine. For Fe-rich phlogopite (BE = 1306.8 eV), the binding energy is greater than for tetra-ferriphlogopite (BE = 1305.9 eV), and this is consistent with the bulk composition having greater F-for-OH substitution in Fe-rich phlogopite (F0.35vs. tetra-ferriphlogopite, F0.24 atoms per formula unit).

SURFACE CRYSTAL CHEMISTRY OF PHYLLOSILICATES USING X-RAY PHOTOELECTRON SPECTROSCOPY: A REVIEW

The characterization of freshly cleaved mica surfaces for surface structure and chemical composition was briefly reviewed and focused on surface crystal chemistry using X-ray photoelectron spectroscopy (XPS) and other surface-sensitive techniques. This paper considers micas, which are useful as a first approximation for the behavior of many clay surfaces. Emphasis was given to phyllosilicate XPS binding energies (“chemical shift”), which were described and used to obtain oxidation state, layer charge, and chemical bonding information from the chemical shifts of different peaks. The chemical shift of the Si2p binding-energy to lower values can result from a negative charge increase because of Si4+ replacement by Al3+ and/or Fe3+. The apparent interlayer coordination number reduction from twelve to eight at muscovite and tetraferri-phlogopite (001) surfaces was indicated by the XPS measured K2p binding-energy and is consistent with bond relaxation. Although chemical shifts are valuable to distinguish chemical bonding and oxidation state, chemical shifts usually cannot distinguish between different Al coordination environments where Al is in both tetrahedral and octahedral sites.

Crystal chemistry and surface configurations of two polylithionite-1M crystals

Abstract This paper explores the crystal chemical features of the bulk and the outermost (001) surface layers of two trioctahedral Li-rich mica-1M (space group C2) polytypes, i.e., a polylithionite (MLG-114) from Li-mica granitic pegmatite at St. Austell (SW England) and a Fe2+-rich polylithionite (Ch-140) from a rhyolite at Profitis Ilias, Chios Island, Greece. Structural formulas are [xii](K0.952Na0.019Rb0.019) [vi](Al1.034 Li1.459Fe2+0.389Fe3+0.046Mn0.038Mg0.002Zn0.002Ti0.001) [iv](Al3+0.477 Si3.523)O10.081(F1.735OH0.184) and [xii](K0.992Na0.014) [vi](Al0.980Li1.028Fe2+0.787Fe3+0.022Mn0.059Mg0.052Zn0.010Ti0.024) [iv](Al3+0.857Si3.143) O10.095 (F1.617OH0.288) for MLG-114 and Ch-140, respectively. Each mineral is characterized by a high F content in the anion site and has tetrahedral and octahedral compositions related to the exchange vector [vi]Li-1[iv]Si-1[vi]Fe2+[iv]Al. Unit-cell dimensions are a = 5.251(1), b = 9.066(2), c = 10.087(2) Å; β = 100.694(5)° for polylithionite MLG- 114 and a = 5.282(1), b = 9.121(3), c = 10.080(3) Å; β = 100.764(5)° for Ch-140. Crystal structure refinements (agreement factors are R = 3.58% and 3.75% for MLG-114 and Ch-140, respectively) demonstrate that the [vi]Li-1 [vi]Fe2+[iv]Si-1[iv]Al exchange vector produces a decrease in the lateral dimensions of the tetrahedral and octahedral sheets. The decrease in basal oxygen distances results from the effect of the strain caused by the orientation of opposing tetrahedral sheets within a 2:1 layer. The decrease reduces the strain so that the basal oxygen plane can remain nearly planar. Changes in these dimensions via distortions of the tetrahedral basal-oxygen ring (α = 3.3° and 4.1° for MLG-114 and Ch-140, respectively) are limited. Octahedral M1 and M3 sites are similar in size and much larger than M2 and the mean electron count is M3 < M1 < M2 in MLG-114 and M1 ≈ M2 < M3 in Ch-140. Al preferentially occupies the M2 site, whereas Fe and Li are nearly disordered between M1 and M3 sites with a slight preference of Fe for the M1 site in MLG-114 and for M3 site in Ch-140. Element concentrations on the (001) surface, obtained through X‑ray photoelectron spectroscopy (XPS) high-resolution spectra for Si2p, Al2p, Fe2p, K2p, Li1s, and F1s core levels, indicate that a greater amount of lithium and a smaller amount of potassium characterize the surface with respect to the bulk. The decrease in K content, commonly observed in micas, is related to its location on the cleavage surface, because the cation must be distributed equally between the two (001) surfaces generated upon cleavage. The increase in Li content on or near the (001) cleavage surface suggests a preference for cleavage near lithium-enriched regions. The surface structure of the polylithionite crystals suggests that Al, Li, and Fe cations maintain coordination features at the surface similar to the bulk. Silicon, however, which is generally in fourfold coordination, shows also a small number of [1]-fold coordinated components at a binding energy of 99.85 eV.

An alternative method of calculating cleavage energy: The effect of compositional domains in micas

Abstract Cell parameters and atomic coordinates for the true micas are varied to simulate layer deformation along the [001]* direction by an external force. The resulting (deformed) structures are then used to determine bonding forces and to calculate a maximum force component along the [001]*. Bonding forces are compared to experimental observations of bond lengths of the interlayer, octahedral, and tetrahedral sites. Calculated bonding forces are consistent with experimental observations that locate the cleavage plane along the interlayer. Because many studies have shown that the chemical composition of the cleavage surface often differs from the structure of the bulk, compositional variations were considered in determining cleavage energy. The chemical composition of the cleavage surface may produce a reduction in cleavage energy. This reduction in energy depends on various elements occurring in greater number at the cleavage surface than in the bulk. A reduction in cleavage energy occurs if there is a reduction in the interlayer site size, as measured by the area defined by the first-coordination basal oxygen atoms. In addition, a reduction in lateral cell dimensions and an increase in the bonding force between the basal oxygen atoms and the interlayer cation also results in a reduction in cleavage energy in the direction normal to the layer. Joins considered are phlogopite-annite, tetra-ferriphlogopite-tetra-ferri-annite, polylithionite-siderophyllite, muscovite-celadonite, and muscovite-paragonite. A lack of homogeneity in composition may produce preferential cleavage locations within the family of (001) planes. The cleavage energy appears to be greater for homogeneous synthetic micas compared to natural micas.

Trioctahedral Fe-rich micas: Relationships between magnetic behavior and crystal chemistry

Abstract Six Fe-bearing mica samples with different Fe ordering, Fe2+/(Fe2++Fe3+) ratio, octahedral, and tetrahedral composition were studied. Four micas belong to the phlogopite-annite join (space group C2/m), two are Mg-rich annite and two are Fe-rich phlogopite, one is a tetra-ferriphlogopite (space group C2/m) and one is Li-rich siderophyllite (space group C2). Thus these samples had a different environment around the Fe cations and layer symmetry. These micas were characterized by chemical analyses, single-crystal X‑ray diffraction, X‑ray absorption spectroscopy, and magnetic measurements. In samples with Fe mostly in octahedral coordination, dominant magnetic interactions among Fe atoms are ferromagnetic, which reach a maximum at higher Fe2+/(Fe2++Fe3+) ratios. Samples with higher Fe2+/(Fe2++Fe3+) ratio are also characterized by higher values of the Curie-Weiss θ constant. Where Fe2+/(Fe2++Fe3+) ratios decrease, θ values also decrease. The Fe3+-rich phlogopite shows predominant Fe3+ in tetrahedral coordination and shows anti-ferromagnetic interactions with a negative value of the Curie-Weiss θ constant (i.e., θ = -25 K). Fe ordering in octahedral trans- and in one of the two cis-sites accounts for a greater θ value in Li-rich siderophyllite when compared to other samples showing similar octahedral Fe content. Our data suggest that Fe3+ cations and other non-ferromagnetic cations hinder long range magnetic ordering. This observation may produce for the different role of octahedral Fe magnetic interactions that can in principle develop along long Fe-rich octahedral chains, when compared to tetrahedral-octahedral interactions that are confined within the layer by the non-ferromagnetic cations of the interlayer. Spin glass behavior is indicated by the dependency of the temperature to produce maxima in the susceptibility curve. These maxima are related to the frequency of the applied AC magnetic field.

Layer charge and heavy metals structures in hydrated 2 : 1 silicates: state of the art and new advances on cadmium

Abstract This study will discuss how layer charge can affect chemical speciation and topology of heavy metals adsorbed to 2 : 1 layer silicates, by providing: i) an overview of literature data; ii) experimental data on Cd complexes adsorbed by 2 : 1 layer silicates with different layer charge (montmorillonite and vermiculite); iii) a comparison between our results and literature data. This study will also be supported by several different experimental techniques such as chemical and thermal analyses, X-ray powder diffraction and X-ray absorption spectroscopy. Based on our data Cd atoms were found to complex water molecules in both clay minerals and to show four-fold coordination in montmorillonite (Cd–O distances of 2.24 Å) and six-fold coordination in vermiculite (Cd–O distances of 2.16 and 2.28 Å). Furthermore our models clearly suggest that Cd mainly bonds to interlayer water, without neglecting the more limited, but still significant, Cd multinuclear surface complexes at the octahedral broken edges. Both clay minerals show H2O/Cd ratio, as evidenced by thermal analyses, drastically higher than expected from X-ray adsorption spectroscopy data, thus implying that most of the water molecules are only loosely coordinated to interlayer cations.